A PI induced accumulation of the CFTR precursor as observed in cultured human cells, was also observed with epoxomicin and MG-132 in our system. This may by itself not be sufficient for functional correction, but is likely show synergy with a suitable folding corrector and activator. Moreover, is seems likely that stabilisation of corrected human F508del CFTR at the apical membrane also occurs. Together this may increase the level of correction to levels required for clinical benefit. Indeed, SCH772984 Gentzsch et al.showed, most recently in a model of differentiated human airway epithelial cells that surface derived normal and F508del CFTR is recycled through endocytosis. The mutant form is recycled less efficiently, and is retained and subsequently degraded in sub-apical vesicles. Moreover, the internalization and lysosomal breakdown of this surface pool of mutant CFTR could be largely prevented by incubation with PI’s. In a related study it was shown by Lukacs et al. that improperly folded domains in F508del CFTR are recognised at the plasma membrane by the Hsc70/Hsp90 chaperone system, which allows CHIP mediated ubiquitination, internalisation and finally degradation of mutant CFTR in the lysosomes. The mechanism by which PIs cause retention of mutant CFTR at the cell surface is still unclear but may involve inhibition of the ubiquination step initiating the internalization of F508del CFTR at the apical membrane, possibly by depletion of free ubiquitin. Recent data by Maiuri et al. redefine CF as a deficiency of autophagy. They show that abnormal trafficking of human F508del CFTR in cell culture and murine F508del CFTR can be effectively rescued by ROS scavengers and inhibition of transglutaminase, which increases the efficacy of CFTR activators. Future studies will be required to establish the effect of PIs on this process, which would offer an alternative explanation for PI induced rescue of mutant F508del CFTR CFTR. A further alternative explanation for the functional rescue of F508del CFTR in native intestinal epithelium was also considered, i.e. PI stimulation of ER stress and of the unfolded protein response, possibly resulting in cell surface trafficking of core-glycosylated CFTR through an unconventional pathway. While we can not completely rule out this explanation, several findings argue against the operation of this mechanism under our conditions. First, ALLN has no apparent effect on the steady-state level of F508del CFTR band B, whereas functional rescue was observed. Further, all PI’s tested increase the level of F508del CFTR band C, in clear contrast to the absence of such effects under conditions of ER stress. Finally, a twofold increase of band B observed following exposure to the PI inhibitors Epoxomicin and MG-132 was not paralleled by an enhanced functional rescue of F508del-CFTR in comparison with ALLN treatment. Additional experiments including a more detailed analysis of PI effects on the peripheral quality control system for mutant-CFTR in native mouse epithelium, and studies of F508del-CFTR rescue by PIs in native human intestinal epithelium are needed to confirm our concept and to explore the applicability of PIs for the rescue of F508del-CFTR processing and function in CF patients. Proteasome inhibitors interfere with multiple cellular processes. Therefore, their therapeutic usefulness for the treatment of CF might be questioned. However, the recent development of peptide boronates made the clinical use of proteasome inhibitors feasible. Preclinical studies in vitro and in vivo using the proteasome inhibitor PS341 demonstrated its antitumor activity in models for hematological malignancies and solid tumors. PS341 can be used effectively at low concentrations, and clinical trials conducted in patients with refractory hematological malignancies reported antitumor activity with manageable treatment related problems. Patients with advanced solid tumors also benefit from PS341-based combination therapies. More recently, promising results were reported in an animal model with MLN9708, a molecular variant of PS341 with better bioavailability. Our data show that PS431 treatment increased the cAMP dependent chloride secretion in F508del murine intestine, and a distinct increase in apical CFTR antigen, with no significant effect on glucose response or tissue morphology. Further development of proteasome inhibitors for long term clinical applications, is therefore not only a priority in the field of oncology but also in the field of CF.